Fair Value Measurement (Quantitative Information Regarding Level 3 Valuations) (Details) $ in Thousands	Sep. 30, 2021 USD ($)	Dec. 31, 2020 USD ($)
Assets:
Estimated Fair Value	$ 3,904,136	$ 2,457,531
Level 3
Assets:
Equity securities	2,569	0
Other liabilities	$ 24,000	0
Level 3 | Stochastic Model/Discounted Cash Flows | Minimum
Assets:
Other Liabilities, Measurement Input	0.00%
Level 3 | Stochastic Model/Discounted Cash Flows | Maximum
Assets:
Other Liabilities, Measurement Input	10.00%
Level 3 | Stochastic Model/Discounted Cash Flows | Weighted Average
Assets:
Other Liabilities, Measurement Input	8.00%
Level 3 | Comparability Adjustment | Minimum
Assets:
Equity securities measured at fair value	0
Level 3 | Comparability Adjustment | Maximum
Assets:
Equity securities measured at fair value	0.10
Level 3 | Comparability Adjustment | Weighted Average
Assets:
Equity securities measured at fair value	0.05
Corporate debt, limited observable inputs | Level 3
Assets:
Estimated Fair Value	$ 19,712	3,265
Corporate debt, limited observable inputs | Level 3 | Comparability Adjustment | Market Comparable Securities | Minimum
Assets:
Assets measured at fair value	0
Corporate debt, limited observable inputs | Level 3 | Comparability Adjustment | Market Comparable Securities | Maximum
Assets:
Assets measured at fair value	0.05
Corporate debt, limited observable inputs | Level 3 | Comparability Adjustment | Market Comparable Securities | Weighted Average
Assets:
Assets measured at fair value	0.025
Corporate debt, limited observable inputs | Level 3 | Comparability Adjustment | Discounted Cash Flows | Minimum
Assets:
Assets measured at fair value	0
Corporate debt, limited observable inputs | Level 3 | Comparability Adjustment | Discounted Cash Flows | Maximum
Assets:
Assets measured at fair value	0.05
Corporate debt, limited observable inputs | Level 3 | Comparability Adjustment | Discounted Cash Flows | Weighted Average
Assets:
Assets measured at fair value	0.025
Residential mortgage-backed securities | Level 3
Assets:
Estimated Fair Value	$ 4,235	2,032
Residential mortgage-backed securities | Level 3 | Comparability Adjustment | Market Comparable Securities | Minimum
Assets:
Assets measured at fair value	0
Residential mortgage-backed securities | Level 3 | Comparability Adjustment | Market Comparable Securities | Maximum
Assets:
Assets measured at fair value	0.05
Residential mortgage-backed securities | Level 3 | Comparability Adjustment | Market Comparable Securities | Weighted Average
Assets:
Assets measured at fair value	0.025
Residential mortgage-backed securities | Level 3 | Comparability Adjustment | Discounted Cash Flows | Minimum
Assets:
Assets measured at fair value	0
Residential mortgage-backed securities | Level 3 | Comparability Adjustment | Discounted Cash Flows | Maximum
Assets:
Assets measured at fair value	0.05
Residential mortgage-backed securities | Level 3 | Comparability Adjustment | Discounted Cash Flows | Weighted Average
Assets:
Assets measured at fair value	0.025
Other asset-backed securities | Level 3
Assets:
Estimated Fair Value	$ 11,007	$ 6,661
Other asset-backed securities | Level 3 | Comparability Adjustment | Market Comparable Securities | Minimum
Assets:
Assets measured at fair value	0
Other asset-backed securities | Level 3 | Comparability Adjustment | Market Comparable Securities | Maximum
Assets:
Assets measured at fair value	0.05
Other asset-backed securities | Level 3 | Comparability Adjustment | Market Comparable Securities | Weighted Average
Assets:
Assets measured at fair value	0.025
Other asset-backed securities | Level 3 | Comparability Adjustment | Discounted Cash Flows | Minimum
Assets:
Assets measured at fair value	0
Other asset-backed securities | Level 3 | Comparability Adjustment | Discounted Cash Flows | Maximum
Assets:
Assets measured at fair value	0.05
Other asset-backed securities | Level 3 | Comparability Adjustment | Discounted Cash Flows | Weighted Average
Assets:
Assets measured at fair value	0.025